Control devices in motor vehicles are, to an increasing extent, continuously supplied with voltage (also called “terminal 30” capability) so that certain monitoring and control functions can be performed even with the ignition switched off. These can be instances of, for example, access and driving authorization, or diagnosis. To reduce power consumption, the control devices are put into a so-called “sleep” mode. This is done either by switching off the voltage regulator or by entering into a corresponding operating mode of the microcontroller.
The control device must be woken up when necessary. This occurs by way of a lead, provided for the purpose, either to a wakeup input of the microcontroller of the user or to a wakeup input of the voltage regulator. In present-day systems, which are usually networked, it can also occur by way of an activity on the bus lines.
A disadvantage of this is that either a separate wakeup lead must be routed to all required control devices; or in the case of waking via the bus, all the control devices (including those not required) are woken by a desired or undesired bus activity, either by communication on the bus or by interference on the bus.
Present-day CAN transceivers can be operated substantially in two modes: in the active mode for communication, and in the sleep mode for the power-saving idle state. The application program can set the desired operating mode; the transceiver also switches, in particular automatically, from the sleep mode into the active mode as soon as it detects a signal or a signal property, e.g., a dominant bit, on the CAN bus. While the transceiver is in sleep mode, the remainder of the CAN node can be switched off and is then switched back on when the CAN transceiver switches into the active mode.
German Patent Application No. DE 103 58 584 A1 describes a method by which a CAN transceiver is expanded to include a circuit that decodes 8-bit-long patterns from an 8-byte-long data field of a CAN message. As a result, the CAN transceiver can be operated in four modes. In addition to the modes previously described, there is also a further economy mode and an intermediate mode. The transceiver automatically switches from the sleep mode into the economy mode as soon as it detects a dominant bit on the CAN bus. In the economy mode, it senses a signal property and switches into the intermediate mode when it detects a number of signal properties, for example edges, within a predefined time span. It then switches from the intermediate mode into the active mode only as soon as it detects a specific pattern in the 8-byte-long data field of the re-transmitted CAN message, which it decodes for that purpose according to a specific method. This wakeup pattern can be configured separately for each CAN transceiver. The CAN identifier of this message is predefined for this method. If it does not detect the wakeup pattern in this message, it goes back into economy mode. The power consumption in the intermediate mode is only slightly higher than in the economy mode; the remainder of the CAN node can remain switched off. This makes possible CAN networks in which individual nodes remain in controlled fashion in the power-saving economy mode, while the other nodes communicate via the CAN bus. The individual nodes can be selectively woken up from the economy mode. This is also called “selective wakeup.”
It is thus possible, via the bus used in motor vehicles (in particular a CAN bus), to selectively wake up only the control devices that are needed in order to perform the necessary functions.